A caterpillar drive type construction machine such as a bulldozer or the like including a direct transmission is constructed such that power generated in an engine is transmitted to left-hand and right-hand caterpillars via a main clutch, the direct transmission and leftward and rightward steering clutches.
The direct transmission has a so-called dog clutch incorporated therein so as to select individual speed stage. When a speed changing operation is to be performed in the direct transmission, first, the main clutch is released from the operatively engaged state thereof to operatively disconnect the transmission from the engine. Thereafter, a speed changing lever is actuated such that a dog clutch corresponding to the speed stage which has been presently selected is released from the operatively engaged state thereof and another dog clutch corresponding to a speed stage to be subsequently selected is brought in the operatively engaged state thereof.
As is well known, the caterpillar drive type construction machine is generally subjected to a large magnitude of resistance against its movement. For this reason, once the main clutch is released from the operatively engaged state to perform a speed changing operation, movement of the construction machine is interrupted within a short period of time. Accordingly, with a conventional construction machine of the foregoing type, there arises such a malfunction that movement of the construction machine is unavoidably interrupted at every time when a speed changing operation is performed.
In a case where a speed changing operation is performed to shift to a low speed stage (shift-down speed changing operation), the construction machine is not required to move at a high speed. Thus, there does not arise any malfunction even when there appears an occasion that movement of the construction machine is temporarily interrupted.
On the contrary, a speed changing operation for shifting to a high speed stage (shift-up speed changing operation) is usually performed in a case where the construction machine is required to move at a high speed. However, interruption of movement of the construction machine during the speed changing operation degrades workability of the construction machine. Additionally, the phenomenon that movement of the construction machine is interrupted during the shift-up speed changing operation leads to the result that movement feeling to be received by an operator is deteriorated remarkably.
The present invention has been made with the foregoing background in mind.
Therefore, an object of the present invention is to provide a method of controlling a speed changing operation wherein particularly, a shift-up speed changing operation can smoothly be performed without an occurrence of interruption of movement of a construction machine, without any necessity for manually actuating a main clutch and without any generation of noisy sound due to incorrect meshing engagement of a dog clutch during the speed changing operation.